This invention relates to a numerical control system suited for application to a cylindrical coordinate system-type robot having an axis of rotation, or to a machine tool. More particularly, the invention relates to a numerical control system in which, when transporting a movable element from a starting point to an end point while controlling the axis of rotation, a fixed point on the movable element is capable of being moved approximately along a straight line which connects the starting point and end point.
The rising cost of labor has given rise to a demand for labor-saving measures and for improvements in work procedures. Industrial robots have been put into use to meet this demand and have been very successful in doing so. Particularly outstanding results have been achieved in plants having numerous machine tools where industrial robots are used to perform such simple tasks as changing the workpieces and tools associated with each machine tool. The demand for such robots has grown yearly. FIG. 1 shows an example of an industrial robot which operates on the basis of a cylindrical coordinate system, wherein (a) is a plan view and (b) a side view. In the Figure, numeral 1 denotes a mechanical hand for gripping workpieces or the like when they are to be changed, 2 a wrist which is capable of being rotated (.alpha.-axis) and bent up and down (.beta.-axis), 3 an arm which can be freely extended and retracted (R-axis), 4 a shaft, 5 a casing which can be moved vertically (Z-axis) long the shaft 4 and swiveled from side to side (.theta.-axis) on the shaft 4, 6 a frame, 7 a teaching box for teaching robot motion, 8 a control panel for operator manipulation, and 9 a control unit for storing, in succession, the instructive content based on what is taught by the teaching box 7, such as the operating position (point), the operating speed, and the various services to be performed, and for controlling the motion of the mechanical hand 1, wrist 2,arm 3 and casing 5 in accordance with the instructive content.
With playback-type industrial robots of this kind, service operations are taught in advance by the teaching box 7, and the instructive content (referred to as robot instruction data hereinafter) is stored beforehand in the memory of the control unit 9. Whenever a request for a service arrives from the machine side, a series of the robot instruction data is read out sequentially, and the robot responds by servicing the machine in repeated fashion.
The robot instruction data comprises point information regarding the point at which a service is to be performed, robot operating speed, and service codes which instruct how the robot hand is to be controlled at the point and which instruct also the exchange of signals with the machine tool side. In general, the abovementioned teaching operation proceeds in the following order: (1) establish the memory addresses at which the robot instruction data is to be stored; (2) carry out positioning by a jog feed (manual feed); (3) set the positional information regarding to the point and set the magnitude of the speed command; and (4) set the robot service codes. A series of robot operations with respect to a machine tools is taught by repeating the steps (1) through (4).
Accordingly, as long as there are no problems with the robot control system and mechanisms, and after the positioning of the robot has been completed on the basis of the predetermined operating speed in accordance with the robot instruction data whenever there is a service request, the robot will sequentially execute, in correct fashion, such services as workpiece exchange, cleaning of machining scraps, tool exchange, manipulation of the hand, and the like.
In a robot which uses a cylindrical coordinate system, the robot is taught in such a manner that the fixed point H on the mechanical hand will move from a starting point P.sub.1 to an end point P.sub.2 (FIG. 2(a)). During playback, a linear interpolation is performed for each axis, with the segment from the starting point P.sub.1 to the end point P.sub.2 serving as a single interval. Since the robot motion between these two points includes rotational motion, however, the path followed by the fixed point H is not linear. Accordingly, in order to linearize the path followed by the fixed point H, it is conventional practice to perform sophisticated arithmetic operations which include a conversion of coordinates for each pulse distribution cycle. This has required that a computer having a considerable processing capacity be incorporated within the numerical control device (referred to as an NC hereinafter) or within a robot control device. In other words, with the conventional method an ordinary microcomputer cannot be used. This has led to an increase in the size and cost of the numerical control device or robot control device. The reason why the fixed point on a movable element such as the mechanical hand is moved in linear fashion will now be explained.
FIG. 2(b) is an illustrative view for a case where a workpiece WK is loaded onto the chuck CHK of a lathe, and is useful in describing the reason why the fixed point H on the mechanical hand 1 must be moved in linear fashion. Portions identical with those of FIG. 1 are denoted by like reference characters.
Assume that a workpiece WK is to be loaded on the chuck CHK by controlling the movement along each axis of the robot. If the fixed point H on the mechanical hand 1 is moved along the straight line SL, the workpiece WK can be mated with the chuck CHK smoothly and loaded onto the chuck without error. In a case where the fixed point H does not move along the straight line SL, however, the workpiece WK will move toward the chuck CHK from an inclined direction and therefore cannot be loaded onto the chuck in reliable fashion. Furthermore, when the workpiece is to be unloaded from the chuck it cannot be gripped with assurance, resulting in cases where the workpiece is dropped in midcourse. Thus it is far more advantageous if the fixed point on the movable element such as the mechanical hand can be moved in a linear manner. The advantages are not limited to the loading and unloading of the workpiece as described above.